I. Field of the Invention
The present invention relates to a trawl net layer-by-layer collecting device, which can collect nektonic animals in the sea according to water depth, by towing a plurality of trawl nets, and by opening and closing net mouths depending on the water depth so as to switch the trawl nets for collecting animals.
In particular, the present invention relates to a trawl net layer-by-layer collecting device, which has a lightweight structure.
In addition, the present invention relates to a trawl net layer-by-layer collecting device of an autonomously openable and closable type, which can autonomously open and close the trawl nets depending on the water depth and the net-towing time period, without being remotely controlled from the ship.
II. Description of the Related Art
With a view to sustainable effective utilization of marine organism resources, the TAC (Total Allowable Catch) system and the TAE (Total Allowable Effort) system, which are intended for major fishes to be fished, have been adopted.
In order to improve a management effect of resources by these systems, it is necessary to exactly assess the resource amount for each kind of fishes. As one of the means therefor, larval and juvenile fishes are collected according to water depth.
In order to collect larval and juvenile fishes in each layer, there have been conventionally proposed a trawl net layer-by-layer collecting device referred to as “MOCNESS” (MOCNESS-type trawl net layer-by-layer collecting device), a trawl net layer-by-layer collecting device referred to as “RMT” (RMT-type trawl net layer-by-layer collecting device), and a trawl net layer-by-layer collecting device referred to as “LOCHNESS” (LOCHNESS-type trawl net layer-by-layer collecting device).
FIG. 8 generally shows a conventional LOCHNESS-type trawl net layer-by-layer collecting device. FIG. 9 shows a longitudinal section thereof.
As shown in FIG. 8, the conventional LOCHNESS-type trawl net layer-by-layer collecting device includes a rectangular cylindrical frame 22 for towing a plurality of trawl nets 21a, 21b, 21c, 21d. 
The frame 22 includes therein plural pairs of vertical bars 23.
A horizontal slider 24 is extended between each of the pairs of vertical bars 23, the slider 24 having opposed ends thereof that are slidably engaged with the vertical bars 23.
As apparently shown in FIG. 9, a net-mouth edge of a lower net of the first trawl net 21a is anchored to the slider 24 between the first pair of vertical bars 23, and a net-mouth edge of an upper net of the first trawl net 21a is anchored to another slider 24 between the second pair of vertical bars 23.
Anchored to the slider 24 between the second pair of vertical bars 23 are the net-mouth edge of the upper net of the first trawl net 21a, and a net-mouth edge of a lower net of the second trawl net 21b. 
Similarly, anchored to the slider 24 between the third pair of vertical bars 23 are a net-mouth edge of an upper net of the second trawl net 21b, and a net-mouth edge of a lower net of the third trawl net 21c. This is similar to the fourth and fifth sliders 24. A net-mouth edge of an upper net of the trawl net and a net-mouth edge of a lower net of the trawl net adjacent thereto are anchored to the same slider.
The LOCHNESS-type trawl net layer-by-layer collecting device 20 has, on an upper part thereof, locking devices 25 for locking the respective sliders 24.
Outside the frame 22, floats 26 are disposed on opposed sides of the frame 22, so that a buoyancy of the LOCHNESS-type trawl net layer-by-layer collecting device 20 can be adjusted.
Horizontal fin stabilizers 27 are disposed rearward the frame 22, so that a stance of the LOCHNESS-type trawl net layer-by-layer collecting device 20 can be stabilized.
Net-towing wires 28 are connected to the upper parts of the LOCHNESS-type trawl net layer-by-layer collecting device 20.
As shown in FIG. 9, in the LOCHNESS-type trawl net layer-by-layer collecting device 20, the first slider 24 is lowered at first. Under this state, the first trawl net 21a is opened, while the other trawl nets 21b, 21c, and 21d are closed.
Then, the second slider 24 is unlocked by the locking device 25 so as to be lowered by its own weight. Thus, the first trawl net 21a is closed and the second trawl net 21b is opened.
Similarly, when the third slider 24 is unlocked by the locking device 25 so as to be lowered by its own weight, the third trawl net 21c is opened. When the fourth slider 24 is unlocked by the locking device 25 so as to be lowered by its own weight, the fourth trawl net 21d is opened.
The locking devices 25 are remotely operated from the ship. In order to control the locking devices 25 from the ship, it is necessary that the ship and the LOCHNESS-type trawl net layer-by-layer collecting device 20 are connected to each other by a conducting wire, or that a transmitter and a receiver of ultrasonic hydrophone are provided.
However, since weights of the conventional “MOCNESS-type trawl net layer-by-layer collecting device”, the “RMT-type trawl net layer-by-layer collecting device” and the “LOCHNESS-type trawl net layer-by-layer collecting device” are heavy, a special winch for controlling a conducting wire or a remote controller formed of an ultrasonic hydrophone are required, but the number of ships capable using these equipments is limited.
Therefore, an object of the present invention is to provide a lightweight trawl net layer-by-layer collecting device, which can collect nektonic animals in the sea according to water depth.
In addition, the conventional “MOCNESS-type trawl net layer-by-layer collecting device”, the “RMT-type trawl net layer-by-layer collecting device”, and the “LOCHNESS-type trawl net layer-by-layer collecting device” have to be controlled from the ship in order to switch the opening and closing of the trawl nets.
When the trawl nets are controlled through a wire from the ship, a special winch and a conducting wire of a cable (armored cable) are needed. When the trawl nets are wirelessly controlled, a transmitter and a receiver formed of an ultrasonic hydrophone are needed. Namely, the ship has to be provided with some equipments for the remote control.
Thus, another object of the present invention is to provide a trawl net layer-by-layer collecting device, which can autonomously open and close the trawl nets, by detecting the water depth and the net-towing time period, under conditions suitable for collecting animals, without any special equipment for controlling the opening and closing of the trawl nets.
In the conventional “MOCNESS-type trawl net layer-by-layer collecting device”, the “RMT-type trawl net layer-by-layer collecting device”, and the “LOCHNESS-type trawl net layer-by-layer collecting device”, each of the net mouths is opened and closed by means of the weight of the slider. Thus, in order that the slider can smoothly slide down, the weight of the slider has to be increased. Such an increase in weight of the slider increases the weight of the overall device.
Thus, a further object of the present invention is to provide a trawl net layer-by-layer collecting device, which is lightweight but is capable of switching the opening and closing of the net mouths by means of the weight of the slider.